1. Field of the Invention
This invention relates to an improved process to prepare plasticized PVC when plasticized with high molecular weight, ethylene copolymer plasticizers. More particularly, it relates to a process improvement, to reduce gel commonly present, which involves addition of small amounts of partially crosslinked ethylene copolymer modifier to the mix of PVC and the (non-crosslinked) high molecular weight, ethylene copolymer plasticizer. The partially crosslinked ethylene copolymer modifier is based on an ethylene copolymer similar in composition to that of the high molecular weight ethylene copolymer plasticizer. Conventional low molecular weight PVC plasticizers are preferably also present as part of the modifier.
2. Description of Related Art
PVC has been plasticized with so called `monomeric` and low molecular weight `polymeric` plasticizers for many years. Low molecular weight `polymeric` plasticizers are really oligomeric polyester materials which are liquid at ambient temperatures.
Some time ago, certain high molecular weight ethylene copolymer resins were also found to plasticize PVC. Typically, these ethylene copolymers are ethylene/vinyl acetate bipolymers containing a high level of vinyl acetate, and terpolymers of ethylene, vinyl acetate or an alkyl acrylate, and carbon monoxide. The mount of the monomers vinyl acetate or alkyl acrylate in the termonomers is considerably lower than vinyl acetate in the bipolymer because the strongly polar nature of the carbon monoxide enhances miscibility, reducing the other comonomer requirement for miscibility. These ethylene copolymers are soft but essentially non-fluid at ambient temperatures.
The above carbon monoxide containing ethylene terpolymers were first disclosed in U.S. Pat. No. 3,780,140 (Hammer). They are disclosed as useful for blending with PVC and other polymers. Two types of `compatible` mixtures are described, one where the terpolymer toughens but does not significantly plasticize, the other where the ethylene terpolymer is miscible on a molecular scale and plasticizes, i.e. reduces the modulus, of the PVC or other polymer. The blends claimed were PVC blends. Blend compositions of this type have been commercial for many years. It is this type of plasticized PVC which the improved preparation process of the present invention is concerned with. This patent also discloses the use of mixed high molecular weight and `monomeric` or low molecular weight oligomeric, liquid plasticizers.
U.S. Pat. No. 5,278,236 (Case et al.) discloses especially useful high molecular weight polymeric plasticizers for PVC which are ethylene/alkyl acrylate/carbon monoxide polymers made under special process conditions which are believed to produce low levels of chain branching. PVC blends with this polymer give smooth surfaces in moldings. Ethylene/n-butyl acrylate/carbon monoxide is especially preferred.
U.S. Pat. No.4,6 13,533 (Loomis et al.) discloses partially crosslinked elastomeric compositions which are blends of PVC and ethylene terpolymers containing carbon monoxide. The ethylene terpolymers which become crosslinked are similar to the ones of concern in the present invention. The crosslinking is typically carded out on the PVC blend, though it may be carried out on the ethylene terpolymer before blending with the PVC. Because of the level of crosslinking in the overall blend, and because of the particular phase structure, the product has properties associated with a vulcanized rubber, particularly low compression set. As a result of the level of crosslinking and phase structure also, the compositions typically exhibit no flow in melt index tests. However, because of the phase structure they can be made to flow under certain conditions. Thus they can be melt processed by extrusion, calendering and injection molding. As such these materials fall into the well-known category of thermoplastic elastomers.
U.S. Pat. No. 4,627,993 (Loomis) describes similar elastomeric compositions which are blends of PVC and crosslinked ethylene bipolymer copolymers without carbon monoxide, but higher level of comonomer in the copolymer. The compositions, likewise, typically exhibit no flow, but can be melt processed.
While the blends described in the above two patents are somewhat similar, chemically, to ethylene copolymer plasticized PVC, the presence of crosslinking makes them quite different in their physical behavior. Thus ethylene copolymer plasticized PVC has relatively high melt flow and poor (high) compression set--just the opposite of the Loomis thermoplastic elastomers.
The ethylene copolymer plasticizers themselves, while non-fluid at ambient temperatures, characteristically have low viscosities (corresponding to very high melt flow) at the melt processing temperatures of PVC. PVC has relatively high melt viscosity and hence low melt flow. The amount of plasticizer used in plasticized PVC ensures adequate melt flow, though not as high as the plasticizer itself.
Plasticized PVC is typically made using PVC which is granular, i.e., having relatively small particles. During preparation of ethylene copolymer plasticized PVC, melt-mixing of the relatively low melt viscosity ethylene copolymer plasticizers and the relatively high melt viscosity PVC can present the typical problems associated with mixing high and low viscosity fluids, namely non-uniformity of mixing, absent extremely rigorous mixing conditions. Thus, while plasticization implies miscibility, the ease of achieving completely uniform mixtures is another matter entirely. In fact, unless very high torque is used to provide high shear energy, it is common to have a high number of undissolved (i.e. unmixed or `unfused` or `unfluxed`) PVC gains or particles. When film is made of the plasticized PVC, the grains appear similar to gel spots common in many polymer films. These particles result in decreased gloss and decreased tensile strength in parts and film made of the plasticized material. Unfortunately, commonly, producer-processors who prepare such plasticized materials have relatively low-intensity mixing equipment, so the problem of undissolved grains of PVC or `gel` is very common.
One approach to reducing this gel has been simply to use higher viscosity ethylene copolymer plasticizer. This approach is described in an article entitled `Plasticization of PVC ethylene copolymer resins`, published in the Journal of Vinyl Technology, March 1994. However, this is not a totally satisfactory solution, and limits the range of plasticizers which can be used.
There remains a need for a process which can reduce the problem of incipient gel, in plasticized PVC, to enable a wider range of plasticizers, process conditions, and processing equipment to be used, and to allow a wider range of plasticized compositions to be utilized.